2003 2.0 gti - New battery now it won't start - help!!!!!

[2001gti]

Hi people

Ok, battery had been on the way out for a while but I bought a new one today and fitted it. Engine turns over very strongly of course but though it fired straight away it was only running on maybe 2 cylinders. Hard to get the revs up, stalls at idle and definitely undriveable.

So ... it's been sat there in the cold and wet for a week and I gave a dusting of WD40 over the spark plug / coil pack and left it half an hour. Now ... it won't fire at all, like literally not even trying, just turning over. Its got fuel in it and was running perfectly until a week ago when it wouldn't start on a cold morning and has been sitting a week. What the f**k is going on?

Please help, I can't fathom a logical reason as to why this would develop by itself. It must be related to the new battery but I can't understand it. It the immobiliser were somehow not being disengaged then it wouldn't have fired on 2 cylinders. I could smell unburnt fuel when it was misfiring so the fuel pump is ok.

What might have happened?

Seriously f**ked off

[2001gti]

Quick update. Was thinking I would check the connection that runs from the fuse box on top of the battery and the positive terminal. Seemed to remember slightly loosening the wrong one of the two nuts on the positive terminal. Turned out i really hadn't loosened it more than 1/10th of a turn but i undid it anyway and agitated the terminals a bit to ensure a good seat then did the nut up dead tight. Whilst doing this I also noticed that the plastic cable plug that plugs into the fuse box had some insulating tape around the 3 wires that run in to the back of it, definitely not a factory item so must have been 'looked at' in some way in the cars life. Also noticed there was some (a few drops) water on these wires around the insulating tape. I dried it off, gave it a bit of wd40 and guess what ....

.... not perfect but it fires on 3 cylinders immediately and could be driven if necessary. I've got it up to operating temperature and turned it off. Plan to let the engine heat permeate round under the bonnet and hope this dries whatever moisture is there. I will try starting it again in half an hour or so.

So .. main panic over, its not a crack case sensor or a failed coil pack and didn't make sense to me that it might be but I've already found a few forum posts with very similar issues that were caused by these failure. It must be one of those 2 things ive just looked at, ie the connection from battery fuse box to the positive terminal or the plastic plug that connects to the back of that fuse box since doing only those two things it went from not firing at all to almost running right.

My instinct is telling me it was those three wires with moisture on them and very old and gaping insulating tape. I think this mainly because the bolt was so close to 'tight' that I can't imagine it would have made any difference. Sure, battery terminals that aren't tight can cause connection issues under heavy load like starter motor but I am guessing that particular connection doesn't deal with anything like the load of a starter motor. I don't know but I am guessing it just routes power to the EMU / coil pack.

Anyway, fingers crossed .....

[2001gti]

Nope .... 14  miles of driving and got nice and hot but still running on 3 cylinders

Is there something I don't know about changing a battery on a golf mk 4? This is mental

[lemski]

Seems an od one. Has it chucked the light on?
Maybe a spark plug has gone down randomely if your sure its not a coil.
Either should chuck a code.
Have you checked that the injector plugs are on.
Compression checked.

Has the earths under the battery tray been checked just incase youve disturbed something by pulling the earth cable....

[2001gti]

Hi cheers for the response mate. Have not done much more with it yet but I think I know the answer ....

.... seems the throttle body needs recalibrating to the EMU .... why changing a battery should make this a requirement staggers me, what a crazy thing to design in to a car, just makes you have to go to a main dealer ... bullsh!t! ... anyway ...

Tried to follow a forum post on how to make the car do this by itself but it wasn't specific to my car (AZJ engine) and although I could hear some motorized actuator or something which seemed like it might be recalibrating the throttle body, it didn't fix the problem, though If i drive now I can use anything up to half thottle and it runs basically mint so perhaps a slight improvement.

So ... lucky for me a mates dad has a pucker VAG computer gizmo I can plug in to it over the weekend and I'm optimistic that this will solve the problem. Found a few posts online about this and one of them (which seemed very knowledgeable) said the car would 'either not run or run very badly' if this recalibration had not been done ... which is exactly what I am experiencing, so ... i'll let you know in a couple of days, fingers crossed!

[2001gti]

Solved - DIY throttle body re-calibration done! Going to post a new thread in the mk 4 section describing it as this must be a common thing. Can't believe disconnecting the battery could cause such havoc with the car!